F= 15cm
 M = 3
 M = \(\frac{v}{u}\)
 V =3u
 \(\frac{1}{f}\) = \(\frac{1}{u}\) + \(\frac{1}{v}\)
 \(\frac{1}{15}\) = \(\frac{1}{u}\) - \(\frac{1}{3u}\)
 \(\frac{1}{15}\) =  \(\frac{2}{3u}\)
U =10cm
 D = u + v
 = 10 +30 = 40cm